In the field of electronics an important goal has been to reduce the size of semiconductor devices and increase the number of electronic components which can be reliably placed on a given semiconductor chip. Recently developed devices such as magnetic bubble circuits have created renewed impetus for manufacturers to develop lithography techniques with improved resolution capabilities for making increasingly smaller features at reduced cost. The aim is to improve lithography resolution to the submicron range, and ultimately to only a few hundred angstroms. D. Maydan, G. A. Coquin, J. R. Maldonado, S. Somekh, D. Y. Low, and G. N. Taylor, "High Speed Replication of Submicron Features on Large Areas by X-Ray Lithography", IEEE Trans on Electron Devices, ED-22, 429, (1975)
Current assembly line techniques use a photo lithography process whereby a polymeric resist on a semiconductor chip is exposed through a mask to visible or ultraviolet wavelength radiation. Practically speaking, these processes are limited to making component features with a minimum size of about 2.mu. (2.times.10.sup.-4 cm). Exposure times are in the range of 20-60 sec. By the process of electron beam lithography, using focused electron beams, devices have been made with features in the submicron range but because the electron beam must be scanned in the desired pattern to expose each spot of resist in the pattern, such processes are inherently slow and expensive. T. H. P. Chang and W. C. Nixon, "Limits of Electron-Beam Nonthermal Interactions", Rec. 9th Symp. Electron, Ion, and Laser Beam Tech., 123 (1967); E. D. Wolf, F. S. Ozdemir, W. E. Perkins, and P. J. Coane, "Response of the Positive Electron Resist Elvacite 2041 to Kilovolt Electron Beam Exposure", Rec. 11th Symp. Electron, Ion, and Laser Beam Tech., 331 (1971); M. Hatzakis and A. N. Broers, "Electron-Beam Techniques for Fabricating Fine Metal Lines", Rec. 11th Symp. Electron, Ion, and Laser Beam Tech., 337 (1971); "Team Produces 80 A Metal Lines", Industrial Research, 19, No. 1, 19 (1977).
X-Ray lithography, because of the small wavelength of x-rays, has the potential to produce resist patterns of extremely high resolution i.e. with a minimum feature size to be measured in angstroms. A resolution of 200 A (200.times.10.sup.-8 cm) represents a factor of 100 improvement in resolution over photo lithography techniques. This means that the density of components in semiconductor devices may theoretically be improved by a factor of 100.sup.2.
X-Ray lithography has the additional advantages of being a one shot process, as opposed to electron beam techniques, and the ability to make deep patterns. This technique has the further, unique advantage of being much less sensitive to dust particles in the manufacturing environment for the reason that the x-rays tend to pass through such particles.
In an x-ray lithography process a substrate having an x-ray sensitive resist layer is exposed through a mask (consisting, for example, of Mylar with a heavy metal pattern deposited thereon) to soft x-rays; i.e. x-rays having wavelengths of greater than about 5 A (5.times.10.sup.-8 cm) or wave energies of less than about 2.48 keV. The resist is developed using a suitable solvent, and steps such as doping of the semiconductor substrate, deposition of conductive layers, or etching of the semiconductor substrate may follow. See: D. L. Spears, H. I. Smith and E. Stern, "X-Ray Replication of Scanning Electron Microscope Generated Patterns", Fifth Int. Conf. on Electron and Ion Beam Tech., 80 (1972); D. Maydan et al supra; Henry I Smith and S. E. Bernacki, "Prospects for X-Ray Fabrication of Si IC Devices", J. Vac. Sci. Technol., Vol. 12, No. 6, November/December 1975; D. L. Spears and H. I. Smith, "X-Ray Lithography--A New High Resolution Replication Process", Solid State Technol., 15, No. 7, 21 (1972). By way of example, the substrate used may be a silicon chip to be made into a semiconductor integrated circuit, or a glass blank to be made into a diffraction grating.
Optimum resolution depends in part on the thickness of metal used in the mask. The method of making very high resolution masks currently in use is electron beam lithography. In making such a mask, Mylar (polyethylene terephthalate film) is coated with a resist which is scanned in the desired pattern with an electron beam. The resist is developed in a suitable solvent and a heavy metal, (usually gold) which is substantially opaque to x-rays is deposited. The mask at this stage has metal deposited on the Mylar in the desired pattern and metal deposited on the remaining resist material which must be removed. A second solvent removes the resist/metal layer leaving only the desired metal pattern on Mylar. In order for the second solvent to work effectively there must be discontinuities in the metal layer at the edges of the pattern. Practical experience has shown that the ratio of the thickness of the metal to the minimum dimension of the pattern ought to be no greater than 1:1. This is known as the aspect ratio. While thinner metal layers permit smaller features, the metal must still be thick enough to effectively block or absorb the radiation. Although x-rays in the 5-15 A range have been used with good success at low cost, R. Feder, E. Spiller and J. Topalian, "Replication of 0.1 .mu.m Geometries with X-Ray Lithography", J. Vac. Sci. Technol., 12, 1332 (1975), because of the above considerations in designing the mask, they are too hard, relatively speaking, to provide optimum resolution.
Because of the softness of the characteristic x-ray emitted, and because of graphite's ability to withstand high temperatures in a vacuum, carbon, with a characteristic wavelength of 44.7 A (50.times.10.sup.-8 cm) and wave energy of 0.277 keV has been recognized as an ideal source for x-ray lithography. R. Feder et al, supra. The ultimate obtainable resolution for carbon sources is about 50 A (50.times.10.sup.-8 cm) but graphite anodes suffer the disadvantages of producing inadequate x-ray intensity. For exposure of a suitable resist such as polymethyl methacrylate (PMMA) with x-rays from a graphite anode, the exposure time is measured in hours. Thus graphite is not well suited for production line manufacturing techniques with the result that very high density semiconductor devices are necessarily very expensive.
In addition to being inadequate x-ray sources for x-ray lithography, carbon sources suffer other disadvantages brought out by attempts to maximize the x-ray output. In producing x-rays, a suitable target is bombarded with charged particles, such as electrons, of sufficient power to cause the target to emit x-rays. Only a small fraction of the energy of the ions is converted to x-rays, the remainder being converted to heat. In order to maximize the power of the charged particle beam being applied to the target, and consequently maximize the intensity of the emitted x-rays, a cooling means is provided for the target to carry away the excess heat. In the case of graphite anodes, this may involve means for rotating the target at high speed and cooling the anode with a fluid such as water. Since the device is operated in a vacuum and the target is rotating, expensive seals are required to keep the cooling water contained in the proper passages and to protect the vacuum. The seals and mechanical equipment tend to be unreliable and expensive.
For x-ray lithography to be competitive, the x-ray source must provide both high resolution and fast throughput at low cost.
It is an object of this invention to provide a device which produces carbon K x-rays of high intensity.
A further object of the present invention is to provide an x-ray device which produces high intensity carbon K x-rays without the need for rotating the anode or for expensive and unreliable coolant seals.
Another object of the present invention is to provide an extremely high resolution method of exposing x-ray sensitive resists.
A further object of this invention is to provide a method for exposing x-ray sensitive resists requiring only a relatively short exposure time.
An additional object of this invention is to provide a very high resolution method of exposing x-ray sensitive resists.
A further object of this invention is to provide a very high resolution method for exposing x-ray resists adaptable to assembly line techniques used in making semiconductor devices.